WL#720: Add support for [table_name].[column_name]

Add support for syntax:
[table_name].[column_name]
when running in sql_mode=MSSQL or SQL_MODE_QUOTE_WITH_BRACKETS
This is needed to make it easy to port sql code from Microsoft SQL server to
MySQL.
In other words, there will be three ways to delimit identifiers:
`with backticks`
"with quotes"
[with brackets]
Since we might support arrays someday, we need to decide
about cases like this:
SELECT a [5] FROM t
Is 5 the index of a, or the alias of a?